I've not got a big family (2 DC,) and I don't know anyone with more than 3 school age children now.
But I used to know quite a lot of big families. 5, 6, 7, DC. Even a couple of 8s.
One family of 6 - (3 boys and 3 girls) in a 3 bed house, used to have 1 bedroom for the 3 girls, 1 for the 3 boys, then the small boxroom for the parents.
One family with 7 DC I knew had a 3 bed 1940s council house with a large lounge, and a small separate dining room. (10 x 9 feet.) The 2 teenage girls (13 and 15) had the small bedroom, the 4 boys aged between 5 and 13 were in the big bedroom, and the parents had the medium sized bedroom. Then the oldest girl (aged 18,) had the small dining room at the back of the house as her bedroom.
I also knew a family with 8 children - 7 girls and 1 boy who had a 4 bed house. The little boy (youngest) had his own little bedroom, (the little 7 X 8 bedroom, and 3 girls had the medium bedroom, and 4 had the second largest, then the parents had the largest one. (Wasn't much between 3 of the bedrooms. They were all fairly decent sizes.)